1. Field of he Invention
This invention relates to zoom lenses of the rear focus type and, more particularly, zoom lenses of the rear focus type for use in photographic cameras or video cameras. Still more particularly, it relates to zoom lenses of the rear focus type having a high range of about 8 to 10 and a large aperture ratio of about F2 in F-number at full open aperture, while nevertheless maintaining a minimization of the bulk and size to be achieved.
2. Description of the Related Art
In the field of cameras for photography and video cameras, a wide variety of zoom lenses having the focusing provision made in other than the first lens unit when counted from the object side, i.e., the so-called rear focus type, have been proposed.
The zoom lenses of the rear focus type have generally advantages over the type of zoom lenses having the focusing provision at the first lens unit, in that the effective diameter of the first lens unit gets smaller, that it becomes easier to minimize the size of the entirety, also that close-up photography, particularly photomacrography, becomes easy to perform, and further that, since the lens unit to be moved is of relatively small size and light weight, because a small power for driving that lens unit is sufficient, rapid focusing can be performed.
A zoom lens of the rear focus type having such advantages is disclosed in, for example, Japanese Laid-Open Patent Application No. Sho 62-247316, where a positive first lens unit, a negative second lens unit, a positive third lens unit and a positive fourth lens unit are arranged on a common optical axis in this order from the object side, the second lens unit being moved to vary the focal length, while the fourth lens unit is moved to compensate for the image shift with zooming and to perform focusing.
In Japanese Laid-Open Patent Application No. Sho 58-160913, a zoom lens has four lens units, of which the first is of positive refractive power, the second is of negative refractive power, the third is of positive refractive power and the fourth is of positive refractive power, the first and second lens units being moved to vary the focal length, while the fourth lens unit is moved to compensate for the image shift with zooming. Among these lens units, one or two or more lens units are moved to perform focusing.
Yet another Japanese Laid-Open Patent Applications Nos. Sho 58-129404 and Sho 61-258217 disclose zoom lenses comprising, from the object side, a positive first lens unit, a negative second lens unit, a positive third lens unit, a positive fourth lens unit and a negative fifth lens unit, wherein the fifth lens unit or a plurality of lens units including the fifth lens unit is or are moved to effect focusing. Another Japanese Laid-Open Patent Application No. Sho 60-6914 discloses a zoom lens of a similar refractive power arrangement to that described above, wherein for a specific infinite object distance, the focusing lens unit takes a constant position on the optical axis independently of zooming.
The use of the rear focusing method in the zoom lens produces, in general case, advantages of getting a lens system of minimized size as a whole and of making it possible to increase the focusing speed, and further of making it easy to extend the focusing range toward the shorter object distances, as have been described before.
On the other hand, however, the range of variation of aberrations with focusing expands, giving rise to a very difficult problem of obtaining a high optical performance throughout a focusing range from infinity to the minimum object distance, while still permitting the bulk and size of the entire lens system to be minimized.
Particularly with the large relative aperture ratio and the high zoom ratio, the problem of obtaining the high optical performance throughout the entire zooming range as well as the entire focusing range becomes very serious.